I’ve noticed that in many of Paul’s projects, he calls for wood pieces 5/8 thick. Has he determined this thickness to be more aesthetically pleasing, or is he starting with 3/4 lumber? I start with 1” thick lumber, and it’s a lot of planing to get down to 5/8 on pieces too wide to resaw.
In the UK it’s pretty common to buy ¾” (18-19mm) machine-planed stock from a DIY store or timber merchant. After hand-planing it would probably end up at about 5/8. Perhaps that is where the dimensions come from.
If you can start with ¾” stock it would definitely save a lot of time, effort and wood!